At the time I was powering it in series-parallel (i.e. 10-ohm) with an iTech-8000 in bridge mode, which is basically dual 5-ohm. About ~8000watts or 800watts per cone.
Obviously underpowered... but I couldn't afford 5 iTech-8000's
I sold all the velodynes, they weren't good enough for me, I had to make my own subs to get something good enough.
I still have the 15's from 2008, but I'm now thinking of selling them and upgrading to another 3 18's. I was thinking of trying some RE XXX-18's
This is as close to a "stack" as it gets.
The best subwoofer I have is the TC LMS Ultra. It has so much magnetic force strength that practically no other subwoofer can complete against it (at least for infrasonic <40hz reproduction); except for maybe an equivalent number of XXX-18's in an IB.
Above 40hz however... pro-woofers are of course going to out SPL it because of the 100db/watt efficiency. That's just the physics of it... what can ya do.